1. Field of the Invention
This invention relates to a film cassette for supporting a film substrate during a gaseous vapor deposition process, to a method for making the cassette, to an apparatus for depositing one or more materials on a substrate using a gaseous vapor deposition process, and to apparatus and methods for loading and unloading the cassettes.
2. Description of the Art
In order to manufacture affordable thin film photovoltaic modules an industrial length roll of an ultra-barrier film (on the order of 10 to 200 meters or more) and about 350-1650 mm in width is needed. An acceptable ultra-barrier film should be able to limit the entry of water vapor and/or oxygen into the photovoltaic layer of a thin film photovoltaic module to a water vapor transmission rate of less than 5×10−4 g-H2O/m2-day. Entry of water vapor or oxygen is deleterious since it tends to rapidly destroy the photovoltaic layer of the module.
Currently, it is possible using a roll-to-roll process to manufacture coated films (such as used for bags of comestible snack products) that have a water vapor transmission rate only as low as 10−3 g-H2O/m2-day. Attempts to use the available roll-to-roll technology to manufacture industrial length rolls of ultra-barrier film for organic light emitting diodes (OLED's) proved unsuccessful, falling far short of the threshold (5×10−4 g-H2O/m 2-day units) necessary for a film to be effective as an ultra-barrier.
In these previous attempts at roll-to-roll manufacturing of coated ultra-barrier film for OLED's a material was deposited onto the surface of a film substrate using chemical or gaseous vapor deposition, such as the process known as atomic layer deposition. During previous roll-to-roll manufacturing attempts, the process rollers contact against the full surface of the substrate, producing surface scratches on the substrate. Moreover, the substrate undergoes significant bending as it is conducted from one roller to another, producing additional cracks through the deposited barrier coating. Such scratches, abrasions, creases or cracks destroy the ability of any deposited barrier coating to prevent moisture or oxygen ingress.
Film cassettes able to support lengths of silver halide film (typically between 35 to 100 mm in width) during chemical batch processing development are known in the photographic arts. Such cassettes typically support the film being developed in a spiral fashion. In a spirally wrapped cassette the film being processed is held edgewise in the spiral groove of the cassette, without the surface of the film being contacted. Representative of such prior art film cassettes are a metallic cassette sold by Hewes Photographic Equipment Manufactures, Bedfordshire, England, and a plastic cassette sold by Paterson Photographic Limited, West Midlands, England.
There are, however, difficulties in scaling either metal wire (stainless steel) or commonly available plastic spiral wound cassettes for use with a film having a width greater than 100 mm.
Although the high rib pitch to interspoke spacing of these silver halide cassettes (about 2.5-6.5%) is ideal for allowing photographic processing liquids to penetrate the spaces between the turns of the spirally wound photographic film, such a high pitch-to-interspoke spacing ratio is very inefficient for processing industrial rolls of film for an ultra-barrier. Only a short length of film is able to be carried on a cassette with such a high rib-pitch-to-interspoke spacing.
Fabrication of metal wire cassettes and low temperature plastic cassettes wider than 100 mm has proven to be difficult since small variations in winding/welding the wire or flow lines in injection molding the plastic cassettes cause distortion of the end plates. These structural distortions would make a film difficult to load. The film would also have a tendency to fall out of the spiral grooves.
Although the metal wire cassettes can take the harsher processing conditions of vapor deposition, their symmetric rib geometry (aspect ratio of 1:1) isn't wide enough to hold the film as it expands from room to processing temperatures, especially for rib pitches less than about 6 mm. The plastic cassettes distort at the harsher processing conditions of vapor deposition, which are well above the heat deflection temperature of the plastic. In addition the self-threading feature of some plastic cassettes creates debris as a film substrate slides along the soft plastic ribs of the cassette.
Accordingly, in view of the foregoing, it is believed advantageous to provide a film cassette that is able to edgewise support a spirally wrapped industrial length roll of a film substrate during a vapor deposition process in a way that minimizes scratching of the film surface during processing and which minimizes risk of creasing or cracking of the film or coating during loading and unloading, thus enabling the manufacture of an industrial length ultra-barrier film.